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  . . . x * * * (row 2, bit value 4) 
  . . . * * * * (row 1, bit value 2)
  . * * . . . * (row 0, bit value 1)

  # swap rows 0 and 1
  . x . . * * *
...
  . * . . * * *
  $args = [ 1, 1, 2, 0, 0, 2, 0, 1 ]
  $height = 2, $v = 1

  # .. and now swap rows 1 and 2
  . . . . * * *
...
  . * * * . . *
  $args = [ 1, 1, 0, 2, 2, 0, 0, 1 ]
  $height = 1, $v = 1

sub rearrange {
  my($dim, $height, $v, $args) = @_;
...
  } 
  ($bitmap[$height], [ @$args[ @vmap ] ]);
}